The classic e-beam system is a "probe-forming" system in which a narrow beam forms the image of the electron source. This source image has a gaussian-like spatial distribution and is scanned over the wafer or other target by an amount equal to a "pixel" at a time, the pixel being defined as the full width at half height of the intensity distribution. Such "Gaussian" systems have the lowest throughput of all probe forming systems due to the serial exposure of patterns one pixel at a time. They have, however, the advantage that corrections can be applied dynamically and pixel by pixel to compensate for aberrations of the electron lenses and deflection units in the system. "Shaped beam" systems have been developed to improve the throughput of the probe-forming systems by projecting a limited number (10-100) of pixels in parallel.
The highest throughput would be obtained with a projection system that projects all pixels in parallel. The classic e-beam projection system is modelled on optical projection systems. In the foreseeable future, chips may have a size of approximately 17 mm.times.35 mm , so that at a typical 4:1 demagnification ratio, the reticle will have a size of 70 mm.times.140 mm. Current technology is unable to produce an electron lens that will cover that size reticle with an acceptable fidelity at a nominal device groundrule corresponding to 0.25 .mu.m or smaller critical dimension.
Throughput is essential if e-beam systems are to compete with light optical and X-ray systems. Therefore, mask projection would be the technique of choice for wafer exposure. A key requirement for high throughput is, of course, a highly intense beam. High power means great heat load on the reticle, which would lead to intolerable distortion of the reticle. An alternative approach to minimize thermal distortion of the reticle of a projection system is that of using a scattering reticle, as described in S. D. Berger & J. M. Gibson, APPL. PHYS. LETTERS 57 (2) (1990) 153, instead of an absorbing reticle. A scattering reticle requires an aperture above the wafer that preferentially absorbs scattered radiation having a greater scattering angle, thus translating scattering contrast into intensity contrast on the wafer.
In addition to covering a large area of the exposed surface, high throughput requires a low dwell time for each area that is being illuminated; i.e. that the amount of charge required to expose the photoresist be deposited in a short time and therefore that the current in the beam be high.
The art has long used a conventional measure called "brightness" and defined as: B=I/(.pi..alpha..sup.2)(.pi..alpha..sup.2), where I is the total current in the beam, is the half angle of the beam envelope (the beam semi-angle) and r is the radius of the beam. As defined, brightness is essentially current density in position-angular phase space and is a constant of the system. The denominator of the brightness is proportional to the square of the emittance of the system (i.e. .alpha.r). The emittance of the beam is the area it occupies in the phase plane. According to Liouville's theorem, the emittance is invariant throughout the system (see, e.g. M. Szilagyi, "Electron and Ion Optics", Plenum Press, NY 1988). Gaussian electron beam systems in the art use electron sources of small dimension (typically 10 .mu.m or less), which are further demagnified by a factor of 10-100 to form an image that is inseparable from the beam source.
In those systems, the source is represented either by the narrowest waist of the beam in front of the emitter or cathode, usually referred to as a "cross-over", since the electron trajectories actually cross over each other, or by the back projection of the individual trajectories emerging from the cathode toward the point of virtual convergence behind the emitting surface ("virtual crossover"). The former is typical for thermionic systems having a beam diameter of the order of 10-30 .mu.m, the latter for field emitter systems of the order of 10.sup.-3 .mu.m.
In shaped beam systems used in the art the shape object (the aperture that determines the shape) is imaged rather than the source. Typical dimensions of the images at the target are in the order of a few .mu.m, divergence angles of the order of a few mrad, and beam currents of a few .mu.A or less. These parameters combine to establish a typical figure of brightness in the art of 10.sup.5 -10.sup.6 A/cm.sup.2 -sr. Since e-beam systems preserve the initial brightness to a good approximation, the beam preserves the initial density in phase space as it forms one or more images of the source.
Along the entire path from source to target, the electrons are subject to Coulomb repulsion, a component of which causes an effect equivalent to a diverging lens and can in principle be corrected, but another component of which causes trajectory displacements by a stochastic process that cannot be corrected. A third component causes spreading of the electron energy distribution leading to chromatic aberrations.
Whenever an image of the object is formed, the interactions are generally increased, so that systems that have a greater number of images of the object will have a greater amount of uncorrectable trajectory displacements than systems that have fewer images of the object, other things being equal.